Magnetic recording media are widely used for recording tape, video tape, computer tape, disc, etc. The recording density of magnetic recording media have become higher and higher, and their recordable wavelength has become shorter and shorter every year. As the recording system for such the magnetic recording media there have been studied analog system as well as digital system. In order to meet the demand for higher density, a magnetic recording medium comprising a metal thin film as a magnetic layer has been studied. However, a so-called coating type magnetic recording medium comprising a dispersion of a ferromagnetic powder in a binder coated on a support is excellent from the standpoint of practical reliability such as productivity and corrosion resistance. As such the coating type magnetic recording medium there has been widely used a magnetic recording medium comprising a magnetic layer containing a ferromagnetic iron oxide, Co-modified ferromagnetic iron oxide, CrO.sub.2, ferromagnetic alloy powder or the like dispersed in a binder coated on a non-magnetic support. However, the coating type magnetic recording medium has a low packing density of magnetic particle and thereby has poor electromagnetic characteristics as compared with the metal thin film type magnetic recording medium.
The enhancement of the electromagnetic characteristics of the coating type magnetic recording medium may be accomplished, e.g., by the improvement of the magnetic properties of the ferromagnetic powder, the smoothening of the surface thereof, etc. Various approaches have been proposed but leave something to be desired for the demand for higher density. The recent trend is for shorter recording wavelength with higher density. This occurs problems of self demagnetization loss during recording or thickness loss during reproduction causing output drop if the magnetic layer is too thick. Thus, an extremely thin film coating type magnetic recording medium has been proposed. In recent years, a so-called ME (metal-evaporated) tape on which a thin metal has been vacuum-evaporated has been put into practical use in the field of Hi-8 and digital VCR for consumers' use. Thus, a system which employs both an alloy powder tape, i.e., so-called MP (metal particulate) and the ME tape has been put into practical use.
In particular, the ME tape which outperforms Hi-8 ME tape is designed as a subreference in the field of DVC, where the format was specified in 1994. The ME tape differs from the conventional Hi-8 ME tape in the composition of magnetic particle. In other words, this ME tape comprises a Co system instead of Co-Ni alloy. Further, this ME tape has a DLC (diamond like carbon) film to protect the metal thin film.
In order to cooperate with ME tape in a DVC system, the MP tapes must have a thinner magnetic layer to attain a higher output similarly to the ME tape. Since the ME tape has a DLC film to protect the hard metal film and the surface layer, it gives a head contact different from that of the MP tape. Further, since the ME tape for DVC gives very little head abrasion as compared with the conventional Hi8 ME tape, the magnetic head is designed to have a smaller effective thickness at the tip thereof in DVC system than in the conventional system.
Heretofore, the MP tape has normally-comprised an abrasive agent having a high Mohs' hardness incorporated in a magnetic layer to minimize the clogging or output drop. However, if such the abrasive agent is incorporated in too large an amount, it presents a problem of aggravation of head abrasion. On the contrary, if such an abrasive agent is incorporated in a reduced amount to reduce head abrasion, clogging or output drop can easily occur.
The applicants have heretofore proposed extremely thin film magnetic recording media comprising a lower non-magnetic layer and a thin upper magnetic layer provided thereon.
For example, the following inventions have been proposed.
JP-B-6-93297 (The term "JP-B" as used herein means an "examined Japanese patent publication") discloses a medium comprising a dispersion of a ferromagnetic powder having an average major axis length of less than 0.3 .mu.m and a crystallite size of less than 300 .ANG. coated on a non-magnetic layer. However, the present invention cannot always provide a less head abrasion than the ME tape and be compatible with the ME tape in head contact.
JP-A-5-298653 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses that a medium having a good overwrite characteristics (capability of being overwritten by 7.6 MHz signal on 1.9 MHz signal) and a less strain during digital recording can be obtained by predetermining the thickness of the magnetic layer to less than 0.3 .mu.m and defining the standard deviation of the thickness of the magnetic layer in a predetermined range. However, no reference is made to the compatibility with the ME tape in head-contact and the head abrasion.
JP-A-5-73883 discloses that an MP tape having performance comparable to that of ME tape, i.e., which exhibits reduced self demagnetization loss at a short wavelength range and attains high output over a range from a low frequency to a high frequency without any space loss due to smooth magnetic layer can be obtained by defining the thickness of the magnetic layer to not more than 1 .mu.m and controlling the variation of the thickness of the magnetic layer. This patent claims that a magnetic recording medium which exhibits a good head contact, excellent storage stability and running durability and less dropout, block error rate and is less liable to edge damage can be provided. However, no reference is made to the improvement in the compatibility with a high performance ME tape comprising a DLC film in head contact.
JP-A-5-28464 discloses a magnetic recording medium comprising a non-magnetic layer composed of only a binder and a semisolid or liquid additive and a magnetic layer having a thickness of not more than 1 .mu.m. This patent claims that the magnetic recording medium can be prevented from having surface roughness to provide improvements in waveform responce, overwritability (capability of being overwritten by 20 MHz signal on 10 MHz signal) and 40 MHz output. However, no reference is made to the compatibility with ME tape in head contact and the head abrasion.
U.S. Pat. No. 5,527,603 discloses that the definition of the proportion of Al and rare earth elements to Fe in a metallic magnetic material provides improvements in electromagnetic characteristics and running durability. Although this patent discloses Co content and the use of Y as a rare earth element, no reference is made to their relationship with the compatibility with ME tape in head contact or the head abrasion. Thus, the objects of the present invention cannot be accomplished by this patent.